Nerve growth factor (NGF) is a diffusible extracellular polypeptide that promotes the differentiation and survival of neurons of the peripheral and central nervous system. The binding of NGF to its receptor, on the surface of target neurons, initiates a remarkable cascade of events that can lead to neuronal cell growth, differentiation, or survival. Fundamental to NGF action is its ability to activate new programs of gene expression. Recent studies have identified a class of immediate early genes (IEGs) that encode mRNAs whose transcription is rapidly and transiently activated when NGF receptor bearing neurons are exposed to NGF. A number of IEGs encode transcription factors that are believed to propagate the NGF signal by controlling transcription of late response genes whose expression is critical for the differentiation response of target neurons. In this application for renewed funding, experiments are proposed that will elucidate the mechanism by which NGF controls expression of one IEG, the c-fos protooncogene, in the rat pheochromocytoma cell line PC12. In PC12 cells, NGF activation of c-fos is regulated by a DNA sequence element termed the serum response element (SRE) that binds the transcription factor SRF. The proposed research will test the hypothesis that NGF activates a protein kinase cascade leading to SRF phosphorylation, and c-fos activation. Among the specific aims of the proposed research are: (1) To determine if SRF becomes modified by phosphorylation upon NGF stimulation, (2) to test the ability of two NGF-inducible nuclear kinases to phosphorylate and regulate SRF activity in vitro (3) to identify SRF-associated proteins that play a role in NGF regulation of c-fos transcription, and (4) to investigate the mechanism of NGF induced repression of c-fos transcription subsequent to its activation. Given the central role that c-fos plays in the control cell growth, differentiation, and cell transformation, these experiments of c-fos regulation are likely to give new insight into the processes of growth factor signal transduction and oncogenesis.